Electronic coin validators include at least one probe which tests a physical property of coins, e.g. the material. Frequently, inductive probes are used, with the damping of the probe being characteristic for a coin. The analogue measuring signal is digitized and then compared with a reference value in order to generate a validity signal for a genuine coin or a rejection signal for a false (counterfeit) or foreign coin. The digital measuring signal normally is processed in a microprocessor which also controls the receipt of a coin and the sorting thereof. The reference value is stored in the memory of the microprocessor.
Usually, more than a single discrete reference value is used for coins of a given denomination because the manufacture of valid coins results in some variance in the properties to be tested. Due to the manufacturing of coins in different years, the wear of the coins, and their contamination, the band of acceptable tolerances must considerably increase.
It is desirable to accept all valid colds in vending machines, and therefore an upper and a lower reference value are determined for each coin denomination, forming a so-called acceptance band. If the measuring signal is within the acceptance band, a validity signal is generated. Preferably, the reference values of an acceptance band are selected such that the measuring signals of all valid coins of a given denomination fall within the acceptance band.